This invention relates to digital communication systems and more particularly to an asymmetric duplex modem for communicating at higher speeds over voice-grade, dial-up telephone circuits utilizing narrow band echo cancellation.
Modem designers are continually seeing new ways to increase the transmission speeds of modems. Although arbitrarily high transmission speeds can be attained over special-purpose, wideband transmission circuits, such links are typically unavailable and transmission must accordingly be handled by conventional, voice-grade, dial-up telephone lines. Such standard phone lines, however, have severely limited bandwidth, are subject to line noise, and typically exhibit line irregularities and poor termination, making them echo-prone. These factors severely complicate the task of improving modem transmission speeds over dial-up lines.
Better use of the roughly 3,400 Mertz bandwidth available on the dial-up network has been realized by using advanced modulation and equalization techniques. Modems capable of operating at high speeds, such as those conforming to the CCITT V.32 Bis standard, achieve this by using a modulation method called "quadrature amplitude modulation" (QAM), in which both the amplitude and phase of the signal are modulated. It would, however, normally be impossible for a high speed QAM modem to simultaneously transit in both directions at the designed speed over voice-grade lines if a technique, called "adaptive equalization," were not used to automatically adjust the modem to the unique characteristics of each phone line encountered.
Another more sophisticated and advanced technique used for high-speed transmission is "trellis coded modulation" (TCM). Trellis coded modulation relies on a special transmit encoding process, which allows transmission at rates that would normally prove unreliable on non-TCM modems. The receiving modem has sufficient intelligence to correct many transmission errors, allowing the modems to communicate at higher speeds for a given error rate. Using trellis coded modulation and adaptive equalization techniques, transmission speeds of 9600 bits per second and higher can be attained.
Another important technique, echo cancellation, allows the forward and reverse channels to overlap. This scheme is found in the CCITT V.32 recommendation for a full-duplex 9600 bit per second modem in which the modem simultaneously receives information over the same frequency band on which it is transmitting. The same scheme is found in the CCITT V.32 Bis recommendation. For a modem to simultaneously receive information over the same frequency band on which it is transmitting, however, each modem must be able to substantially cancel out the "echoes" of its own transmitted signal. Echoes are reflections of the transmit signal typically caused by a discontinuity in the transmission path of the signal, such as an impedance mismatch. V.32 and V.32 Bis use a method called "echo cancellation" to eliminate the reflected transmitted signal. This method involves subtracting a locally synthesized replica of the reflected transmitted signal from the composite received signal. High-speed modems utilizing the CCITT V.32 Bis standard are known to those of ordinary skill in the art and are commonly available. Such modems are made by several firms, including Motorola, Rockwell, AT&T, Penril, General Datacom, Racal Datacom, and U.S. Robotics.
Schemes such as V.32 and V.32 Bis have significant disadvantages, however. One of these disadvantages is due to the fact that such schemes utilize symmetrical data transmission. In symmetrical data transmission, the same bandwidth is used in both the forward and reverse directions, regardless of the relative data traffic requirements. Accordingly, the bandwidth of the echo is the same in both directions, regardless of relative data traffic. This is undesirable for several reasons. First, because of the relatively broad bandwidth of the echo in symmetrical transmissions, modems must achieve a high degree of echo cancellation to substantially eliminate the unwanted echo. Second, it requires a high degree of precision digital arithmetic in the Digital Signal Processor of the modem. Third, separating a wide-band echo from the local receiver signal requires considerable computational power. In addition, separating a wide-band echo from the local receiver with a full power local transmitter requires a high degree of precision in the analog-to-digital and digital-to-analog converters of the modem. Modems utilizing symmetrical transmission spend approximately the same computational power on echo cancellation as on signal demodulation and decoding. Furthermore, the residual echo that cannot be cancelled becomes a major limitation to the local receiver, even if the local transmitter has little or no data traffic, which is often the case. Finally, the V.32 and V.32 Bis standards require the same data rate in both directions. Thus, if two modems are connected over a channel that is more impaired in one direction than the other, the data rate in both directions is limited to the maximum rate in the more impaired direction. Hence, the direction with the lowest channel speed determines the maximum speed for both directions.
Much of the data traffic from one data terminal to another is highly asymmetric in nature. In recognition of this, modems have been developed utilizing asymmetrical data transmission techniques in which the data rates in the forward and reverse directions differ. Asymmetrical transmission is based on the recognition that, while the modem needs to operate interactively in both directions at once, the high-speed data transfer is normally needed in only one direction at a given time. When entire files are being transferred from one location to another, a high-speed channel is needed in that direction while, in the other direction, the data to be sent is normally limited to the combination of the interactive data being keyboarded and the error-control signals needed to confirm the accuracy of the transmission occurring in the high-speed direction.
Examples of modems utilizing asymmetric data transmission to achieve higher transmission speeds are found in U.S. Pat. Nos. 5,008,901 and 4,890,316. In these modems, simultaneous bi-directional transmission occurs over a wide-band, high-speed channel in one direction and a narrow-band, low-speed backchannel in the reverse direction, and the direction of the high-speed channel is dynamically reversed whenever the modem currently transmitting over the backchannel accumulates more than a predetermined maximum backlog of untransmitted data. These inventions use band splitting techniques to achieve the simultaneous, bi-directional transmission of the data. Under band splitting, each channel has its own assigned band of frequencies. Because the channels are non-overlapping, the receiver can use analog or digital filters to separate the received signal from the echo. In such a system, an echo cancellation mechanism is not necessary. The drawback to using band splitting, however, is that it takes part of the already limited available bandwidth of a dial-up telephone circuit, and assigns it to the low-speed channel. Thus, the bandwidth available for use by the high speed channel is reduced.